The present invention refers to a method and a device for detecting patterns on a travelling substrate, in particular on a substrate being deposited on a foil or sheet matter.
Such substrates usually consisting of stamping foils. The substrates are intended for being deposited on sheet or foil matter, which are, for example, used in the packaging industry for applying metallic foils on blanks or on any other sheet or foil material. That operation can typically be carried out either in a platen press, or a rotary press or any other processing machine, by means of a stamping foil and printing plates that are, together with the processable sheet or foil matter, pressed against one another between upper and lower tools of the press or of the processing machine. Under the pressure generated in the presses or processing machines, a stamping foil portion is thus diecut and deposited by heat-sealing on the sheet or foil matter. U.S. Pat. No. 5,486,254 describes in more detail one of the presses and namely a device enabling registering pre-printed patterns on a foil with patterns of a stamping press.
There are plenty of types of stamping foils, among which are the reflective plain specular foils, the holographic diffraction foils and the reflective structured foils. The latter are of iridescent and variable effect offering patterns more or less visible depending on the angle at which one observes them.
The regular plain foils are usually uniform from one end to the other and do not comprise any specific pattern. They can thus be processed into the press without worrying about a specific printing mark location. Only an optimal use of the matter aiming to reduce the waste will be the main feature taken into account for defining the foil travel.
For hologram foils, the machine operator must be able to ensure that the foil travel is perfectly controlled so that the patterns, i.e. holograms, are at any time in perfect register with the printing plates applying them on plate elements. Even a slight shift is not allowed because the pattern would thus be altered or it would cause an eccentric application of the hologram.
when using so called structured foils, it is advisable to ensure that foil portions are contrarily not applied by the patterns. Such foils effectively comprise connections regularly distributed each 50 or 60 cm, for example. The connections depend on the foil manufacturing embodiment itself. They are issued from the decorative metallic layer printing on the supporting foil. Since printing is processed in a rotary press equipped with a cylindrical printing plate supporting the geometrical structure to print, this produces a fine transverse line of the order of one or a few tens of mm. width to appear on the foil at each revolution of the printing cylinder. That connection is directly issued from the cylindrical printing plate which has a pattern that can obviously not cover the entire 360° of the cylinder.
Such foil connections can also appear on holographic foils. When the space between two connections is not related to a pitch multiple of the foil holograms, it is quite obvious that one holographic pattern will unfortunately step over a connection. When manufacturing high quality packaging, it is not admissible that a holographic pattern crossed by a connection is deposited on a packaging box blank. In such a case, one will have to switch the holographic pattern and deposit the next one.
Using the word pattern, one understands here and in the whole following specification, that it can either be a hologram pre-printed on the foil, or a foil connection or a transition between two substrates of different structure, as well as a register mark or even the imprint left by the pattern on the plate-like element. From time to time, one ensures that the pattern, i.e. a holographic pattern, is in perfect register with the printing plates, although in other cases, one will on the contrary ensure that the pattern, i.e. a foil connection, is effectively never stamped by the patterns on plate-like elements.
It is known to use foil scanning devices enabling detecting, before stamping, the threading of such a pattern and then consequently modifying the stamping foil travel. Such scanning devices comprise at least one foil lighting unit, an objective and a photoelectric sensor which translates the intensity of the light reflected by the foil into an answer signal.
The goals of the devices are conventionally simple and have each foil portion pitch to be examined under a different angle varying also according to the measure spacing, i.e. according to the distance between the objective related to the foil plan. The scanning devices are also equipped with one or two symmetrical lights, external to the measuring optical and bent with respect to the optical axis of measure. If such devices are well adapted for scanning diffusing foils wherein the light is precisely diffused into the entire solid angle formed by the two lights, they become on the other hand inappropriate for scanning specular foils wherein the light is reflected apart the opening of the measuring optical.
In other cases, the lighting device external to the measuring optical comprises a semi-transparent mirror arranged at 45° in front of the objective, as well as a source of light located perpendicularly to the measuring axis. The source of light, partially reflected by the blade, lights the foil according to the same axis than the optical measuring system. If the substrate is metallized, the reflected light travels back in the direction of the objective, one part reflected by the semi-transparent blade is lost, and the second part enters the objective and enables measuring. When using diffusing substrates, a large part of the incident light is diffused in all directions after reflection and only a very tiny part is sent back towards the objective. That returning part is not or only very little depending on the light incidence angle. It is thus not necessary for the light to be in the same axis than the axis of the measuring optical. One will rather try to get a light of a maximum intensity which is often easier to obtain with an indirect light.
With matrix systems, there are lighting devices associating at the same time a direct light, perpendicular to the foil plane, and an indirect light, arranged crosswise according to that plane. However, as those lighting devices are very large in dimensions, the sensors including at the same time a measuring unit as well as a lighting unit comprise a simple measuring objective, as previously described, as well as a less performing lighting system, either of indistinct type or direct type. The photoelectric sensors connected to these cameras are intended to deliver images of a foil portion with a surface usually equivalent to several thousands of pixels. Such sensors generate thus a huge quantity of images data which are hard to process because of short times.
Among the various sensors known to date, one notices that the ones having the best signal-to-noise ratio with structured foils, become unreliable when using other foil types with specular effect. No sensor today enables treating efficiently and reliably all foil types, either metallized or not, diffusing, structured, refracting or diffracting. The preferred solution today aims to provide at least two sensors in the platen press, one dedicated, for example, to structured foils and the other more specifically to all other foil types. It is then necessary to manually switch over to one or the other sensor according to the type of foil used. Because the average cost for such a sensor is already almost high, such a solution is thus economically not very convenient.
Another drawback of the known devices consists in that the choice of a preferred light in accordance with the substrate used will often depend on the know-how of the machine operator as well as on results of multiple tests. Moreover, the answer quality delivered by the sensor will also vary with respect to the angular positioning of the lighting devices and the scanning device according to the foil plan. A bad setting will not enable reaching a sufficiently contrasted signal-to-noise ratio ensuring detecting the threading of a requested pattern, all the more when the foil bottom is structured and comprises for example a plurality of false interconnected pattern.
With the recent use of structured foils, it is particularly difficult for the current sensors to differentiate the foil connection from an edge of a geometrical shape constituting the structured foil bottom. When the latter is checkered, for example, it becomes particularly difficult to detect a pattern, like a foil connection, since the signal-to-noise ratio delivered by the sensor is not sufficiently well-marked.